1. Field of the Invention
This invention relates to noise cancellation, and in particular to a noise cancelling system, and to a method for controlling a noise cancellation system. More specifically, the invention relates to adaptive feedforward noise cancellation systems.
2. Description of the Related Art
GB-2441835-A discloses an adaptive feedforward noise cancellation system, in which ambient noise is detected by an ambient noise microphone. The electrical signal representing the ambient noise is then applied to a feedforward filter, which generates a noise cancellation signal. The noise cancellation signal is passed to a loudspeaker, which generates sound based on the noise cancellation signal, with the intention that the sound based on the noise cancellation signal should be equal in amplitude but opposite in phase to the ambient noise reaching the user's ear, so that destructive interference takes place, and the ambient noise is at least partly cancelled.
An error microphone is provided, for detecting the sounds reaching the user's ear, namely the combination of the ambient noise and the sound based on the noise cancellation signal, and one or more characteristics of the feedforward filter are adjusted based on the signal received from the error microphone.
More specifically, GB-2441835-A recognises that, if the gain applied by the feedforward filter is too low, then the error microphone will detect some residual ambient noise. The effect of this will be that the signals generated by the ambient noise microphone and the error microphone will be substantially in phase. By contrast, if the gain applied by the feedforward filter is too high, then the ambient noise will be overcompensated so the effect of this will be that the signals generated by the ambient noise microphone and the error microphone will be substantially in anti-phase. GB-2441835-A therefore discloses that a determination as to whether the signals generated by the ambient noise microphone and the error microphone are in phase or in anti-phase can be used to control the gain applied by the feedforward filter to bring it to an optimum value.
However, this analysis assumes that the phase delay through the feedforward filter path does in fact match that required in order to cancel the acoustic ambient noise, so that the error microphone signal is indeed exactly in phase or exactly out of phase with the ambient noise microphone signal. If the frequency response characteristic of the electrical and/or acoustic feedforward paths includes some unanticipated phase delay, the effect of the control method described above is to try and minimise that component of the sound generated in response to the noise cancellation signal that is in phase with the ambient noise, regardless of any quadrature component. Thus, the noise cancellation is not optimised. One situation in which this can arise is in cases where, in order to remove wind noise from the ANC processing, there is a high-pass filter, which will introduce a consequent phase lead in the lower audio band, even above the corner frequency.